In drilling wells for oil and gas exploration, understanding the structure and properties of the associated geological formation provides information to aid such exploration. Measurements in a borehole are typically performed to attain this understanding. However, the environment in which the drilling tools operate is at significant distances below the surface and measurements to manage operation of such equipment are made at these locations.
Logging is the process of making measurements via sensors located downhole, which can provide valuable information regarding the formation characteristics. For example, induction logging can utilize electromagnetic signals that can be used to make measurements. As an electromagnetic field penetrates into a conductive medium, the electromagnetic field can experience a loss in amplitude and change in phase. This loss and change in phase can be referred to as the skin effect. In an induction log, the skin effect causes a reduction of both of the R-phase (in-phase) and the X-signal (out-of-phase) signal at a receiver of the induction tool.
Accurate correction of skin effect plays an important role in wireline induction log data processing. For skin-effect correction (SEC), multi-frequency SEC methods using only R-signal data and single-frequency SEC methods using both R-signal and X-signal data are widely used in the logging industry. The multi-frequency SEC methods typically do not work very well if only one-frequency signal is available. An R-signal is a resistive signal that is a portion of an alternating signal at a receiver of an induction logging tool such that the resistive signal is in phase with the transmitter current of the induction logging tool. The R-signal depends on formation conductivity, where a signal out-of-phase with the transmitter current is a reactive signal, referred to as an X-signal. The X-signal also depends on formation conductivity, in a manner different from the R-signal.
Examples of induction logging systems that can operate at multiple frequencies include Halliburton's Hostile Array Compensated Resistivity Tool (HACRt™) logging system and Array Compensated Resistivity Tool (ACRt™) logging system. HACRt™ has an asymmetric design that consists of a single transmitter operating at three frequencies and six receiver antennas with spacing from 6 to 80 inches. ACRt™ incorporates a transmitter that operates at three frequencies simultaneously with six sub-asymmetrical arrays of antennas strategically spaced from 6 to 80 inches from the transmitter. Further, the usefulness of such measurements may be related to the precision or quality of the information derived from such measurements.